1. Field of the Invention
The present invention relates to a voltage level shifter, and more particularly, to a voltage level shifter with voltage boost mechanism for providing an interface between two circuit units having different operating voltage swings.
2. Description of the Prior Art
Historically, the primary mode of reducing power consumption in electronic circuits has been to insistently scale down the power supply voltage. Recently, a move to 1.8 V power supply has been popularized among low-power and high-speed circuit designers. However, problems may arise when a low-voltage integrated circuit is coupled to a high-voltage integrated circuit, or when the output signal of an integrated circuit having a low operating voltage swing is utilized for driving another integrated circuit having a high operating voltage swing. That is, when a front-end integrated circuit having a low operating voltage swing is coupled to a back-end integrated circuit having a high operating voltage swing, the operating voltage swing of the output signal generated by the front-end integrated circuit is required to be converted from the low operating voltage swing to the high operating voltage swing by making use of a voltage conversion interface.
Please refer to FIG. 1, which is a circuit diagram schematically showing a prior-art voltage level shifter with voltage boost mechanism. As shown in FIG. 1, the voltage level shifter 100 comprises a first transistor 111, a second transistor 112, a third transistor 121, a fourth transistor 122, and an inverter 131. The supply voltage for use in the inverter 131 is a first voltage Vdd1. A second voltage Vdd2 is furnished to both the sources of the first transistor 111 and the third transistor 121. The first voltage Vdd1 is also furnished to the gate of the fourth transistor 122. The second voltage Vdd2 is greater than the first voltage Vdd1. When the input voltage Vin of the inverter 131 is a ground voltage, the internal voltage Vint is the first voltage Vdd1. Alternatively, when the input voltage Vin of the inverter 131 is the first voltage Vdd1, the internal voltage Vint is the ground voltage. The first transistor 111 through the fourth transistor 122 are coupled to form an inverting circuit. When the internal voltage Vint is the ground voltage, the output voltage Vout generated by the inverting circuit is the second voltage Vdd2. On the contrary, when the internal voltage Vint is the first voltage Vdd1, the output voltage Vout generated by the inverting circuit is the ground voltage. That is, the first operating voltage swing regarding the input voltage Vin falls into a range between the first voltage Vdd1 and the ground voltage, and the second operating voltage swing regarding the output voltage Vout falls into a range between the second voltage Vdd2 and the ground voltage.
Accordingly, the voltage level shifter 100 functions to perform a voltage level shifting operation on the input voltage Vin having the first operating voltage swing for generating the output voltage Vout having the second operating voltage swing. However, both the first voltage Vdd1 and the second voltage Vdd2 are required in the circuit operation of the voltage level shifter 100. For that reason, a voltage generator is normally installed for providing dual supply voltages to the voltage level shifter 100, and therefore the voltage level shifter 100 is hard to be put into applications. In other words, when the voltage level shifter 100 is used for interfacing two circuit units having different operating voltage swings, the voltage level shifter 100 is unable to perform the voltage level shifting operation based on only a single supply voltage of either circuit unit.